The present invention relates to an improved non-contact type pattern sensor, and more particularly relates to improvement in construction of a pattern sensor such as a magnetic encoder in which one or more detecting elements such as magnetic resistor elements are arranged facing the pattern track on a recording medium such as a magnetic disc or tape.
In the case of such a non-contact type pattern sensor, the clearance between the pattern track on the recording medium and the detecting element tends to fluctuate due to low working and/or mounting precision of the recording medium as well as due to age warping of the recording medium after long use. Such fluctuation in clearance has a negative influence on the output of the sensor. In particular, the resulting distortion of the output wave shape and the variation in output level greatly lowers the SN ratio, degree of dissolution and precision in output.
In order to remove such negative influences resulting from fluctuations in clearance, a unique non-contact type pattern sensor has already been proposed. In the case of this pattern sensor, a pair of magnetic resistor elements are arranged side by side facing the lower face of a rotary recording media and a pair of magnetic resistor elements are arranged facing the upper face of the recording media also. These four magnetic resistor elements form a bridge circuit in combination with a pair of ordinary resistor elements. Drive voltage is applied to the input terminal and an output voltage is taken out from the output terminals. The sensor changes its resistance on detection of the pattern stored in the recording media. This sensor takes the form of a bridge circuit including a variating section made up of the magnetic resistor elements whose resistances change in response to the fluctuation in clearance and a constant section made up of the ordinary resistor elements of constant resistances.
It is assumed with this sensor that the resistances of the magnetic resistor elements at zero magnetic field is equal to R, the change in resistance corresponding to the maximum fluctuation in clearance is equal to .DELTA. R.sub.max, the change in resistance caused by detection of the pattern on the recording medium is equal to K.sub.1 and K.sub.2, and the resistance of the ordinary resistor elements is equal to 2 R. When the lower side clearance is minimum and the upper side clearance is maximum, the resistance of one lower side magnetic resistor element is equal to K.sub.1 (R+.DELTA. R.sub.max), the resistance of one upper side magnetic resistor element is equal to K.sub.1 (R-.DELTA. R.sub.max), the resistance of the other lower side magnetic resistor element is equal to K.sub.2 (R+.DELTA. R.sub.max) and resistance of the other upper side magnetic resistor element is equal to K.sub.2 (R-.DELTA. R.sub.max).
When the maximum degree of resistance change by detection of the stored pattern is 4%, K.sub.1 is equal to 1.04 and K.sub.2 is equal to 0.96. Then the voltage V.sub.1 between the one lower and upper magnetic elements and the voltage V.sub.2 between the other lower and upper magnetic elements are given by the following equation: EQU V.sub.1 =2.08R.multidot.V/4R=0.52V EQU V.sub.2 =2R.multidot.V/4R=0.5V
Then, the voltage V.sub.out between the output terminals of the bridge circuit is given by the following equation: EQU V.sub.out =V.sub.1 -V.sub.2 =0.02V
From the foregoing, it is clear that the output of this early proposed sensor is dependent on the voltage only without any influence by the clearance. Despite this merit, the sensor requires use of ordinary resistor elements as outer components and the low output voltage (0.02 V) causes SN ratio problem.